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The evolution of the northern margin of the Tethys in eastern Switzerland

FUNK, Hanspeter, et al.

Abstract

The former northern margin of Tethis is being studied in the Alpine chain by IGCP Project 198("Evolution of the Northern Margin of the Tethys"). This paper summarizes the present state of the knowlege from the opening phase of this Mesozoic to early Paleogene seaway in Eastern Switzerland. Basement rocks and sediments of the previous margin are now exposed here in the stack of the Alps and their European foreland.

FUNK, Hanspeter, et al . The evolution of the northern margin of the Tethys in eastern Switzerland. Episodes , 1987, vol. 10, no. 2, p. 102-106

Available at:

http://archive-ouverte.unige.ch/unige:90541

Disclaimer: layout of this document may differ from the published version.

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_ 犷 T h e   E v o l u t i o n   o f   t h e

U N E S } U     No r t h e r n  Ma r g i n   o f   Te t h y s

i n  East ern  Swi t zerl and

by  Ha ns pe t e r   Funk,   Ro l a nd  Ob e r hd ns l i ,   Adr i a n  Pf i f f ne r ,

       St ef an  Schmi d  and  Wal t e r  Wi l di

The former northern margin of Tethys is being studied in the Alpine chain by IGCP Project 198 ("Evolution of the

Northern Margin of the Tethys"). This paper summarizes the present state of knowledge of the opening phase of this

Mesozoic to early Paleogene seaway in Eastern

Switzerland. Basement rocks and sediments of the previous margin are now exposed here in the nappe stack of the Alps and in their European foreland.

ntroducton

The Alpine Tethys is the oceanic basin that opened by extensional and strike-slip tectonics from Triassic time onward, separating the former Pangaea into the Gondwana

continent in the south and Laurasia in the north.   Thi

marine opening was synchronous with the beginning of the expansion of the North Atlantic basin. However, the Alpine Tethys closed again by orogenic inversion beginning near the end of Early Cretaceous time, some 110 Ma ago. Thus, the former continental margins of the Tethyan basin are now exposed on land, in the outcrops of the Alpine chain (Fig. 1).

Within the framework of IGCP Project 198, European geol- ogists are now seeking a better understanding of the evolu- tion of the northern margin of the Tethys from Triassic to early Paleogene times, which must obviously have been quite different from that of the southern margin. A key area for the northern margin is Eastern Switzerland, where the geological history is particularly well studied. Two main problems arise here: what is the most coherent palinspastic reconstruction of the former paleogeographic realm obtain- able by restoring the Alpine tectonic history; and which

mechanisms of basin subsidence predominated 一extensional tectonics, post-Hercynian thermal relaxation, or strike-slip tectonics.

The Mesozoic to early Paleogene margins and basins of Tethys in Eastern Switzerland are found in several geolog- ical units, (Fig. 1), which are reviewed from north to south in the following. Figure 2A is a synoptic cross section showing the relative position of the various units discussed

n the text.

On the southern margin of the Alpine Tethys (the Austro- alpine realm and Southern Alps of Figs. IB and IC), Meso-

zoic extensional tectonics resuled in tited blocks and

basins, as outlined by Bernoulli and others (1979) and Winterer and Bosellini (1981). The style of this margin has been compared to the tectonic style of the Armorican con- tinental margin of Bay of Biscay, as revealed by reflection seismics (Bally et al., 1981).

The European Foreland

The European Foreland is represented by the fold-and-thrust belt of the Jura mountains, composed of Mesozoic sediments

and he ormer ntracontinental Jura basn. The sediment

are mainly evaporitic and shallow-water Triassic carbonates

and Jurassic m ars and shalow-water lm estones.  Below

this sedimentary cover, the meso-European crystalline base- ment is cut by an east-west, late Hercynian intracontmental

rift, with continental elastics and coal seams.

      

! To he outheast s the M olasse

  

诊 入

理一 -

? I Iberia F] G. I B

匆2

   

Plateau, which constitutes

M iocene foreland basin of

crystalline basement here is

he the

of the Swi

   Oligocene-

   Alps. The overlain by a sequence, which is

Africa

缈l l e l 多粼

o l l -    

Mestaithrustom ewhat tns.edozoiThe M olc sedihinner thanmenasse sedi m ent the Jura m oun-

are slightly

on the Plateau and form  a foland

OUTHERN bel

Alps (left photo).

   at the external

side of Fig. 2A,

m argi and see

of the

   c ov e r

N}C

   100 knn

卜一-------一 ,

乏  

眯 o  Genova

国 口

FUROPEAN MARU NAL PLATFORM 八N D  B八 、IN %

OCEANIC  SOUTH-PENNINEC  REAL 

NORTi卜 八N几 从]n pENNINIC  NO仪TH卜胶N C ON IIN EN TALM 八R G IN  UNHT }

八DRIA, OL丁FERN  CONTINENTAL  MARGI AND  PL八1ORM

Figure 1:   The regional  setting and main geological features of the Alps.

二: Main trends of the Alpine chain in   the   Mediterranean   realm;   B:

Simplified   geological   map   of   the Western   an d   Central   Alps;   C:

Simplified  palinspastic  map  of  the Central   and   Western  Alps  in  Late

Jurassic  tim e.

102 EPISODES, Vol. 10, No. 2, June 1987

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N S

  

me_

Iamb,

10 10

20kn 二二二 20km

o  

1 ) N W

SE

   Sw iss

PLATEAU

   AAR

MASSIF    Vattis

HELVETIC PLATFORM

       Alvier

NORTH一PENNINIC BASIN SOUTH-PENNINIC

a b s e n t   ( s e d i m e n t a r y   g a p . e r o s i o n ? ) __

   下一 ,}  ;x-- 共己三 今 百荟工

舫 TI ADDLA AVERS

叁攀篡彗

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ow ret

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二~ .~~一 一‘二一一升一 ‘ -- - .. -

min  

       clastics ii︶

译︒

一 羚

The Alps

The ancient Tethyan basin and continental margin units of Europe (the northern margin) and of Adria (the southern margin) constitute a pile of thrust sheets (Fig. 2A). Within these nappes, the Mesozoic sediments define three geolog} ical mega-units, namely the Helvetic, the Perminic and the Austroalpme zones. As demonstrated in the following, the northern mar-gin is preserved in the Helvetic zone and in the northern and middle units of the Penninic zone (Fig. 113 and C). The south-Penninic units are "oceanic" basin relicts, and the Austroalpine zone represents the Mesozoic southern margin of the Tethys, which belongs to the Adriatic prom-

ontory of the African continent (Laubscher, 1975, Bernoulli et al., 197 9, Derc ourt et al., 198 5).

In the Helvetic zone, the Hercynian basement of the Aar massif and its sliced-up Mesozoic and Tertiary cover are overlain by the Helvetic nappes (71-iimpy, 1969, Milnes and Pfiffner, 1977, Pfiffner, 1978, 1981). In all units, thin epi- continental sediments of Triassic age are followed by Juras- sic to Eocene platform units. Compression and thrusting in

his zone started in the Late Eocene and contnued into

Middle Miocene time (Pfiffner, 1986). The north-Penninic units are composed

basement nappes (Gotthard, of Alpine crystalline Lucomagno,

Tambo and Suretta, see Fig. 2A)

sequences. The latter comprise

omagno, Simano, Adula,

and of thick sedimentary

m iti Lower

shaly,

Late

sedim ents Cretaceous

of Triassic age,epicontinental and dolo-

   urbidic Jurassic and

shales and arenites (BUndnersehiefer) and

arenitic and Cretaceous t

conglomeratic Early EocenE

The sedim ents are detached from

ment.  Alpine metamo甲hism in

deep-water clasties of age (Pr}ttigau Flysch). their crystalline base- these units is mainly epizonal, with amphibolite facies in the extreme south. The mid-Penninic units consist of thick platform carbonates of Triassic to lowermost Cretaceous age in the north, and Triassic carbonates and Jurassic to Lower Cretaceous scarp breccias in the south. The Upper Cretaceous to Paleogene

deposits are hernipelagic and turbiditic.

EPISODES, Vol. 10, No. 2, June 1987 103 Figure 2: Paleogeographic evolution of the northern

margin of the Tethys in Eastern Switzerland along transects  shown  in  Fig. 1B.   A:  Synoptic  cross section,  indicating the relative position of the various units discussed in the text. B: Paleogeo-

graphic section from the Jura basin to the Penninic basin the Barremian (Lower Cretaceous)。

      

      

      

      

       3匕

       4七

       5名

       6毛

       7 任

       8器

      

      

Figure 3:   Geohistory diagrams of the continental margin units of Eastern Switzerland, showing total subsidence of pre-Mesozoic basement. (a) Jura basin of Eastern Switzerland。 (b) Southern part of the Helvetic platform (Alvier section). (c) Hypothesis for the subsidence of the north-Penninic Pr9ttigau basin.   (d) Mid-Penninic Klippen unit  in Central Switzerland.  e) South-Alpine Generoso basin.

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NW S

巍髦夔彝一

蘸 蘸馥酬夔辨{

SK

蘸 鬓

U 】

轰器派

擎 哪龟扁饭

彝然

Figure 4:   The   SNntis   nappe,   an   allochthonous platform   ni t   in   Eastern   Switzerland。 h  lithological units are laterally very continuous,

and synsedimentary extensional tectonics can only be deduced   from  precise一_measurements   of  sediment thickness.   UJ: Upper Jurassic limestone formation.

SK:   "Schrattenkalk," a   platform   formation   of Barremian to early Aptian age.

sea-level changes and a somewhat irregular basin subsidence

controled the sedim entation.  This was eihpr of qhqlnw

basin type with deposition of marls,

form progradation (Bolliger and Burri,or dominated by plat- 1970, Gygi, 1986). In this basin, no extensional tectonic features such as listric faults or scarp breccias have been observed until now. We, therefore, postulate that subsidence was due to thermal relaxation after a late Hercynian thermal event. Creta- ceous sediments are absent due to erosion and/or a sedi- mentary gap.

It has been suggested that mid-Penninic units exist in two different structural situations (Fig. 2A; Streiff,   1962, TrUmpy et al., 1969). In the first, the Falknis-Sulz flub nappes overlie the north-Penninic units and are themselves overlain by south-Penninic ophiolites. In the second settinLy. ine cnams nappes envelop the font of the Suretta base-

ment   nappe and   overlie the   Avers Biindnerscbiefer containing basic voleanics in blueschist facies. However, as north-south and east-west nappe transport is suspected in this area, the debate is still open   concerning   the paleogeographic origin of the Schams nappes.

The Helvetic platform of eastern Switzerland existed from the Triassic onward, in

continental basin fill,

只 号 r t

   tn e

on the top of a late

"Verrucand' trough Jurassic and Cretaceous sediment sequencesa re

Paleozoi

(Fig. 213).

dom inated

The south-Penninic units in this transect of the Alps are

s l i c e s   o f 下丽币石孤歹妥蔽几n d  o f   t h e  P l a t t a   n a p p e  wi t h  o p h i -

olites, and pelagic and hemipelagic sediments of Jurassic and Cretaceous age. The Austroalpine zone comprises the Silvretta basement nappe and the Mesozoic cover sediments of the Northern Calcareous Alps (Fig. 2A). Deformation in the Penninic zone and at the southern margin of the Tethys

( t h e  Au s t r o a l p i n e  r e a l m)   s t a r t e d  i n  E a r l y  C r e t a c e o u s   t i 币e

and continued into the Paleogene.

by carbonates, grading into a more marly sequence in the southern part of the shelf. The total thickness of Mesozoic deposits is about 1.5 km in the north and 3 km in the south.

Despite the lack of synsedimentary faults and conglomer- ates, extensional tectonics appear to be mainly responsible for the subsidence, because the geohistory diagrams (Fig. 3)

indicate a stepwide subsidence with the characteristics of

e x t e n s i o n a l   ma r g i n s   ( F u n k ,   1 9 8 5 ) .  Mo r e o v e r ,   i s o p a c h  ma p s

and precise palinspastic reconstructions of the sediment prisms illustrate sedimentation in tilted basins from the

Early Jurassic to early Barremian (TrUmpy, 1980, Strasser and Funk, 1977). No evidence of extensional tectonics has

been shown to exist fom  the Barremian OD of Schrat i:enKalK,- rig. 4) to the early Faleogene when global sea- level changes and detrital input were controlling the

QPtiim PnfnH nn

Trassic o Low er Cretaceous Evolution

The whole area that subsided during the Mesozoic to form the northern margin of the Alpine Tethys was underlain by a Hercynian (Variscan) crystalline basement with east-west and northeast-southwest late Bercynian rifts. Since the opening of the marine basins only began in Triassic to Early Jurassic time, and since orogenic inversion of the basins started in the Early Cretaceous, the discussion here of the evolution of the margin is generally limited to this time

nterval

In Early Triassic to Late Jurassic times. the Jura basin krig. ztj) was a slowly subsicing trough together with the northern and central part of the Swiss Plateau. The south-

e r n  p a r t   o f   t h e   P l a t e a u  a n d  t h J蔺石歹万厂王 蔽  Aa r  ma s s i f

emerged during early and middle Liassic (the Alemanic land of TrUmpy, 1949). In the Middle and Late Jurassic, global

104   EPISODES, Vol. 10, No. 2, June 1987

A  p a l e o g e o g r a p h i c  g a p  ( " mi s s i n g  l i n k "   o f   F i g .   2 1 3 )   e x i s t s

between the southernmost outcrops of the Helvetics and the northernmost outcrops of the north-Penninic basin. The Late Cretaceous and early Paleogene Sardona Flysch, an exotic strip sheet within the Helvetic nappe stack, may represent a relict of this area (RUefli, 1959).

The north-Penninic PrMtigau basin began with epiconti- nental and shallow-marine deposits in the

Jurassic, and then subsided rapidly during iassic and Early Cretaceous, as indicated by the turb

Tr Jurass

diic and Early deep-water sedimentation of the BUndnerschiefer (Ngnny, 1948, Pantic and Isler, 1978). In the Valais basin, farther west, coarse

念篇; 黯e i   n L t r i a a s b s a i s c i 群瞬 s 摆e 默 嚼豁篡n e

t h i n k   t h a t   s t r o n g ,   s u b s i d e n c e   a n d 。   b a s e m e n t   t o p o g r a p h y

i n d i c a t e   a   r e g i m e   o f   e x t e n s i o n a l   t e c t o n i c s   i n   t h i s   a r e a .

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When taking into account the presence of helminthoid trace fossils in the Upper Cretaceous flysch sequence of the Prdttigau, one can estimate water depth at that time of several hundred metres. In this case, the total pre-orogenic subsidence of the basin is about 5 to 7 km, and the width after stretching is estimated at 80 to 100 km. Lateral sediment input from the west rather than from the east has to be postulated,for most of the Cretaceous time; Paleocene and Eocene sediment transport directions are from the north

t o  t h e  s o u t h ,   mo s t   p r o b a b l y  f r om  a n  o r o g e n i c  s c a r p  a t   t h e

northern ma馆in, to the Priittigau basin in the South (Nanny,

1948, Allemann, 1957).

sedimentary record. Late Cretaceous to early Paleogene sedimentation was controlled by global sea-level changes and terrigenous sediment input. Strong extensional tec- tonics with subsidence are indicated in the neighbouring basin to the south, the north-Penninic Priittigau basin, with turbiditic deep-water clastics from Jurassic to Early Eocene time (Biindnerschiefer and flysch sedimentation).

The interpretation of the transition from the extensional part of the European margin to the south-Penninic realm wih oceanic crustal charactertics

hypothesis suggests high with platform

Penninic basn from

hat a continuous

  controversial. One east-west structural

In the BUndnerschiefer of the Avers area and overlying the Adula basement nappe (Fig. 2A, B), ultrabasic rocks such as prasinites, gabbros, serpentinites and glaucophane schists

sedimentation separated the north- the south-Penninic realm, indicating

are intercalated in the sediments, indicating volcanic activity

1986).

n the

during basin subsidence (Oberhansli More complete ophiolitic sequencesa re

1977, 1978, only known overlying south-Penninic units (Dietrich,

hus seems that the north-Penninic basement hinned contnental crust.

1967). It represent

As regards the evolution of the presumed m些 里ennin丝 realm, in the Schams nappes the Middle Triassic carbonates

c o n t a i n  s ma l l   l a y e r s   o f   a l k a l i c  a s h - f a l l   t u f f s   ( S t r e i f f   e t   a l . ,

1976), comparable to similar rocks in the Western Alps, which are now being linked to growth faults of Liassic age.

In Middle Jurassic to Early Cretaceous times, the slowly subsiding Schams and Falknis platforms were bordered to the South by a zone of steep scarps, indicating tectonic activity by extension and/or strike-slip movements. The angles of unconformities at the base of the scarp breccias

are up to 4o 。

In the Western and Central Alps, the Briangonnais, the M6dianes- and the Klippen-units are considered as remnants of a Mesozoic structural high or "sweirl 60 to 100 km wide,

south of the north-Penninic basin (Fig. 1C, Boillot et al., 1984A). Nonetheless, it is still possible that these mid- Penninic units may also include the Schams, Falknis and Sulzfluh nappes. For the Falknis nappe, a southern (Austro-

alpine) origin has recently been re-stated (Gruner, 1981). However, this hypothesis is not consistent with the tectonic DOSition of this unit. New investigations by St. Schmid and colleagues inaicate tne possiDiiity oi an eastern origin oi ine Schams nappes with respect to their actual position.

These two alternative paleogeographic solutions are illus- trated in Figure 2B. In the hypothesis of an eastern origin of the Schams nappes (Fig. 213i), the mid-Penninic high was a laterally fragmented ridge, and the north-Penninic basin Dassed in Dlaces continuously by crustal thinning into the south-Penninic basin ee also weissert ana ernouin, iu6o)

In the hypothesis of a mid-Penninic origin of the Schams nappes (Fig. 213ii), a crustal block with moderate crustal stretching was intercalated between the stretched north-

discontinuous stretching, most probably coupled with strike-slip tectonics.   Alternatively, the structural highs formed rather isolated islands, and a continuous transition from one deep basin to the other and from continental crust to oceanic crust may have existed in places. In any case the

margin is not a simple Atlantic type stretched margin. Crustal stretching in the Penninic basins was thus fairly discontinuous, with areas of almost normal crustal thickness in the mid-Penninic structural highs, and strong extensional tectonics in the north-Penninic basin. Continental margin formation was, therefore, quite different from the simple stretching model as published for several Atlantic margins and the southern margin of the Alpine Tethys. This may be due to sinistral strike-slip movements linked to the opening

o f   t h e   Te t h y s ,   a s   p o s t u l a t e d  b y  L e mo i n e《 些 B o i l l o t   e t   a l . ,

1984), corresponding to a large scale strike-slip between the African and the European continents.

Acknowledgements

Research on the evolution of the northern margin of the Alpine Tethys is sponsored by the Swiss National Science Foundation projects 2.107-0.86, 4.901-0.85.20, 4.897-0.85.20

and 2.155"0.86.

H.   Funk   teaches   sedimentary petrography at the Swiss Federal Institute  of Technology   (ETH,

Sonneggstrasse 5, CH- 8092 ZUrich,

Switzerland). He is known for his research work on the stratigraphy,

sedimentology and paleogeography of the Helvetic platform in Eastern

Switzerand.

Penninic basi basin. In thi

nental margin

a rea ca se

and the partly

the extenson

of the TethysW a S

  oceanic south-Pennini

of the European conti- very discontinuous with alternating areas

blocks of almos

Penninic highs).

of strong thinning (Priittigau basin)and no thinning of the continental crust (mid-

R. Oberhhinsli teaches magmatic rocks and ore deposits at the Mineralogical Institute of Bern (CH 3012 Bern, Switzerland). His main activity concerns the geochemistry of volcanics and the geology and geochemistry   of   high-pressure metamorphic rocks in the Alps.

Conclusions

On the European continent in the north, the intracontinental Jura basin and part of the Swiss Plateau were slowly sub- siding from Triassic to Late Jurassic time. However, as no extensional features are known in this basin, subsidence was most probably controlled by post-Hercynian thermal relax- ation, though this is not yet proven by paleogeothermic data.

To the south of the Plateau, the Helvetic realm, which was a 60 km wide European shelf of the Tethys, subsided by moderate crustal stretching (less than 10%), from Early Jurassic to Early Cretaceous time, as indicated by the

A. Pfiffner (Geological Institute,

Universit6 de Neuch6tel, 11 rue Emile-Argand, CH-2000 Neuch6tel, Switzerland) is coordinator of the eastern geotraverse of the Swiss NFP 20 project, aimed at exploring the deep structure of the Alps. His main research s ocussed on rock deformation and Alpine tectonics.

EPISODES, Vol. 10, No. 2, June 1987   105

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